Image reading apparatus for detecting noise in image data

ABSTRACT

An image reading apparatus includes: three line sensors mutually spaced in a sub scanning direction; a platen arranged between the original and the three line sensors; a mover moving the platen relative to the three line sensors at a rate relative to the three line sensors, the rate being different from that of the original relative to the three line sensors; a lightness difference detector extracting a feature pixel having a predetermined feature from each of three data output from the three line sensors; and NOR and AND devices comparing the three data corresponding to a single location on the original to detect the feature pixel extracted from one of the three data, as a noise pixel if the feature pixel is not a feature pixel for the other data.

This application is based on Japanese Patent Application No. 2004-285829filed with the Japan Patent Office on Sep. 30, 2004, the entire contentof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to image reading apparatuses andparticularly to image reading apparatuses reading an original whiletransporting it.

2. Description of Related Art

Conventionally digital copiers and similar image reading apparatusesemploy a technique referred to as so-called “reading an original whilepassing the original.” More specifically, an original is transportedrelative to a fixed line sensor in a sub scanning direction orthogonalto the line sensor as the original is read.

Such image reading apparatus is provided with a transparent platenbetween the original and the line sensor to fix a position at which atransported original is read. The original reflects light which is inturn received via the platen by the line sensor.

As such, if dust, paper particles, flaws or other similar foreignmatters (hereinafter generally referred to as “dust”) adhered on theplaten's reading position, the line sensor will read the dust whilereading a transported original. This provides an output image with noisein the form of a line in the sub scanning direction.

Japanese Laid-Open Patent publication No. 2000-278485 describes an imagereading apparatus that detects noise caused by dust adhering on a platenglass's reading position from a read image by oscillating the platen ina main scanning direction as the apparatus reads an original. This imagereading apparatus detects a specific waveform appearing in an image asnoise generated by reading dust.

The image reading apparatus described in Japanese Laid-Open Patentpublication No. 2000-278485, however, employs pattern-matching to detectthe specific waveform appearing in an image. As such, if an originalincludes such a pattern, the apparatus would erroneously detect thepattern.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above disadvantageand contemplates an image reading apparatus capable of detecting withimproved precision noise generated in an image by dust existing on aplaten.

To achieve the above object the present invention in one aspect providesan image reading apparatus including: at least three line sensorsmutually spaced in a sub scanning direction; a platen arranged betweenthe original and the line sensors; a mover moving the platen relative tothe line sensors at a rate relative to the line sensors, the rate beingdifferent from that of the original relative to the line sensors; anextractor extracting a feature pixel having a predetermined feature fromeach of at least three data output from the line sensors; and a detectorcomparing the data output from the line sensors corresponding to asingle location on the original to detect the feature pixel extractedfrom one of the data, as a noise pixel if the feature pixel is not afeature pixel for the other data.

In accordance with the present invention an original is scanned in a subscanning direction by at least three sensors spaced in the sub scanningdirection and between the original and the line sensors there isprovided a platen moving at a rate relative to the line sensors, therate being different from that of the original relative to the linesensors. When the platen has dust adhering thereon, the dust is read bythe line sensors sequentially. As the platen is moved at a rate relativeto the line sensors, the rate being different from that of the originalrelative to the line sensors, the dust on the platen is read by eachline sensor at a different location in the original. The image readingapparatus compares at least three data corresponding to a singlelocation on the original to detect a feature pixel, extracted from oneof the data, as a noise pixel if the feature pixel is not a featurepixel for all of the other data. The image reading apparatus can detectthe noise generated by dust existing on the platen from an image of aread original with higher precision.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an MFP including an image readingapparatus in one embodiment of the present invention.

FIG. 2 schematically shows the image reading apparatus's internalstructure.

FIG. 3 is a perspective view of a mechanism employed to oscillate aplaten.

FIGS. 4A-4C are diagrams for illustrating a theory of detecting noisegenerated by reading dust from a read image.

FIG. 5 is a rear plan view of the platen.

FIG. 6 shows a position on a platen read by a reader.

FIG. 7 is a block diagram showing a configuration of an image processorof the image reading apparatus in the present embodiment.

FIGS. 8A and 8B represent one example of RGB signal output from thereader.

FIG. 9 is a block diagram showing a configuration of a noise detectionprocessor of the image reading apparatus in the present embodiment.

FIGS. 10A-10F show an edge extraction filter by way of example.

FIG. 11 is a block diagram showing a configuration of the noisedetection processor in an exemplary variation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter reference will be made to the drawings to describeembodiments of the present invention. In the following description, likecomponents are denoted by like reference characters and also identicalin name and function.

FIG. 1 is a perspective view of a multi-function peripheral (MFP)including an image reading apparatus in one embodiment of the presentinvention. With reference to the figure, the MFP includes an imagereading apparatus 10 operative to read an original image, and an imageforming apparatus 20 provided under image reading apparatus 10. The MFPforms an image read by image reading apparatus 10 on a sheet of paper orsimilar recording medium. Furthermore, the MFP includes a communicationsinterface to connect with a facsimile device, a local area network(LAN), a public line or similar network.

FIG. 2 schematically shows an internal configuration of image readingapparatus 10. Image reading apparatus 10 includes an automatic documentfeeder (ADF) 101 and a main body 103. ADF 101 includes an upperrestraint plate 203 guiding a transported original in the vicinity of anoriginal reading position, a timing roller pair 201 transporting theoriginal to the original reading position, and a roller pair 202transporting the original having moved past the reading position.

Main body 103 includes a platen 205 formed of a transparent member, asheet passage guide 207 forming a portion of a path of the original, asource of light 206 illuminating the original at the reading position, areflector member 208 reflecting the light emitted from the source oflight, a reader 213 having three line sensors arranged in a sub scanningdirection, a reflector mirror 209 arranged to reflect light reflectedfrom the original and guide the reflection of light to reader 213, alens 211 focusing the reflection of light on reader 213, an imageprocessor 215 processing an electrical signal output from reader 213, amotor 219 operative to oscillate platen 205, and a motor controller 217operative in response to a control signal received from image processor215 to control the driving of motor 219.

An original 200 is transported by timing roller pair 201 between platen205 and upper restraint plate 203 in a direction D1. The original beingtransported has its image read at a reading position L by reader 213successively. ADF 101 transports an original in the sub scanningdirection, as seen at a reading position L. During the image readingoperation, platen 205 is oscillated by motor 219 in a direction D2.Platen 205 oscillates in a direction substantially parallel to the subscanning direction.

Reader 213 includes three line sensors each having a plurality ofphotoelectric conversion elements arranged in a main scanning directionsubstantially perpendicular to the sub scanning direction. The threeline sensors have filters, respectively, different in spectralsensitivity and receive light reflected from an original through thefilters. More specifically, the sensors have filters transmitting lightof waveforms of red (R), green (G) and blue (B). Thus, the line sensorhaving the filter of red (R) outputs an R signal, an electrical signalindicating an intensity of red light of light reflected from anoriginal, the line sensor having the filter of green (G) outputs a Gsignal, an electrical signal indicating an intensity of green light oflight reflected from the original, and the line sensor having the filterof blue (B) outputs a B signal, an electrical signal indicating anintensity of blue light of light reflected from the original.

The three line sensors are arranged in the sub scanning direction in apredetermined order with a predetermined distance therebetween. In thisexample, the line sensors are spaced by a distance corresponding tothree original reading lines, and arranged, red first, followed by greenand then blue as seen in the direction in which an original istransported, although the line sensors may be spaced by differentdistanced and arranged in different orders.

The three line sensors thus spaced and arranged simultaneously receiveat the same timing the light reflected by an original at differentlocations. As such, the light reflected by the original at a location isinitially received by the red light receiving line sensor, subsequentlyby the green light receiving line sensor, and finally by the blue lightreceiving line sensor. This delay is adjusted by image processor 215, aswill be described later.

Platen 205 is formed of a transparent member, such as a flat plateformed for example of glass, resin or the like.

Note that while in the present embodiment reader 213 is provided withthree line sensors, it may be provided with four or more line sensors.

FIG. 3 is a perspective view showing a mechanism employed to oscillatethe platen. With reference to the figure, platen 205 is held by a platenholder 221 held slidably in the sub scanning direction by a guide 220fixed to the main body of image reading apparatus 10. Platen holder 221has one surface with two arms 222 connected thereto. Arm 222 has theother end provided with a circular hole.

A shaft 224 at portions corresponding to the two arms 222 has two cams223 attached thereto. Furthermore, shaft 224 has one end with a gear 225attached thereto. Gear 225 is arranged to mesh with a gear 226 linked bya belt to the motor 219 drive shaft. As motor 219 runs, the motor'srotation is transmitted by the belt to gear 226, and gear 226 thusrotates. As gear 226 rotates, gear 225 and shaft 224 rotate.

Cam 223 is arranged in the circular hole of arm 222. As such, as shaft224 rotates, the two cams 223 accordingly provide rotation, which isconverted to translation movement of platen holder 221.

Note that platen 205 may be oscillated by a mechanism other than thatdescribed above. For example, the platen may be oscillated by amechanism employing a driving source, such as a piston utilizing anelectromagnet, air pressure, hydraulic pressure and the like, causinglinear movement.

Platen 205 is oscillated parallel to the sub scanning direction. Whenplaten 205 is moving in a direction opposite that in which an originalis transported, platen 205 and the original moves in the oppositedirections. As such, the speed of platen 205 relative to line sensors213R, 213G, 213B and that of the original relative to the line sensorsare different. In contrast, when platen 205 is moving in the directionin which the original is transported, the speed of platen 205 and thatof the original transported are the same in direction. Preferably, theyshould be different in rate. Note that while herein platen 205 isoscillated parallel to the sub scanning direction, the platen may beoscillated in different directions.

In the present embodiment image reading apparatus 10 detects noisegenerated by dust adhering on platen 205 from a read image in accordancewith a theory as described hereinafter. FIGS. 4A-4C are diagrams forillustrating the theory. For the sake of illustration, an original andplaten 205 are transported in the figures in a direction indicated by anarrow, and platen 205 moves at a rate which is the same in direction asand twice in magnitude that at which the original is transported.Furthermore for the sake of illustration the three line sensors are redlight, green light and blue light receiving line sensors arranged redfirst, followed by green and then blue in the direction in which theoriginal is transported, with a distance corresponding to three linestherebetween. R, G and B indicate outputs of the red light, green lightand blue light receiving line sensors, respectively.

FIG. 4A is a diagram for illustrating interline correction. The image ofa portion of the original is initially read by the red light receivingline sensor arranged most upstream in the direction in which theoriginal is transported. The image is then transported by a distancecorresponding to four lines, and read by the green light receiving linesensor. The image is further transported by a distance corresponding tofour lines, and read by the blue light receiving sensor.

Thus an image located in an original at a single location is read bythree line sensors at different times. As such, the three line sensorsoutput signals offset in timing. Interline correction synchronizes thesignals output from the three line sensors so that the signals allcorrespond to a single location in the original. More specifically,output R is delayed by eight lines and output G is delayed by fourlines.

Interline corrected outputs R, G and B are composited to provide acomposite output, which corresponds to outputs R, G and B read at asingle location in an original and composited together.

FIG. 4B is a diagram for illustrating a composite output provided whendust adhering on a platen is read. The dust adhering on platen 205 isinitially read by the red light receiving line sensor arranged mostupstream in the direction in which an original is transported. The dustis transported by a distance corresponding to four lines, and read bythe green light receiving line sensor. Since platen 205 moves in thesame direction as the original at a rate twice that at which theoriginal is transported, the dust moves by four lines in a period oftime required for a line sensor to read the original by two lines. Assuch, between the time point at which the red line sensor reads the dustand that at which the green line sensor reads the dust there isintroduced an offset by a period of time corresponding to reading twolines. Furthermore, the dust is transported by a distance correspondingto four lines, and read by the blue light receiving line sensor. Sinceplaten 205 moves in the same direction as the original at a rate twicethat at which the original is transported, between the time point atwhich the green line sensor reads the dust and that at which the blueline sensor reads the dust there is introduced an offset by a period oftime corresponding to reading two lines.

By interline correction the red light receiving line sensor reading thedust outputs R delayed by eight lines and the green light receiving linesensor reading the dust outputs G delayed by four lines. As such,interline corrected outputs R, G and B composited together provide acomposite output in which outputs R, G and B with the dust read are notcomposited at the same timing, offset by two lines.

Note that the figure shows a composite output provided when paperparticles or similar white dust adhere on platen 205 and a blackoriginal is read. Despite that the white dust is read, the compositeoutput is not white but rather an output of blue, green and red dividedin three lines.

FIG. 4C is another diagram for illustrating a composite output providedwhen dust adhering on a platen is read. The figure shows an example ofreading dust having a size corresponding to ten lines in the subscanning direction. Platen 205 moves in the same direction as anoriginal at a rate twice that at which the original is transported. Assuch, the dust is read as having a size corresponding to five lines.

The dust adhering on platen 205 is initially read by the red lightreceiving line sensor arranged most upstream in the direction in whichthe original is transported. The dust is then transported by a distancecorresponding to four lines, and read by the green light receiving linesensor. Between the time point at which the red line sensor reads thedust and that at which the green line sensor reads the dust there isintroduced an offset by a period of time corresponding to reading twolines. The dust further is transported by a distance corresponding tofour lines, and read by the blue light receiving line sensor. Betweenthe time point at which the green line sensor reads the dust and that atwhich the blue line sensor reads the dust there is introduced an offsetby a period of time corresponding to reading two lines.

By interline correction the red light receiving line sensor reading thedust outputs R delayed by eight lines and the green light receiving linesensor reading the dust outputs G delayed by four lines. As such,interline corrected outputs R, G and B composited together provide acomposite output in which outputs R, G and B by five lines with the dustread are not composited at the same timing, offset by two lines. Notethat the figure shows a composite output provided when paper particlesor similar white dust adhere on platen 205 and a black original is read.Despite that the white dust is read, the composite output is an outputvarying in color, first in blue, followed by cyan, white yellow and thenred.

The dust adhering on platen 205 is thus divided in an image into aplurality of lines, which are extracted for each color as a featurepoint to detect noise. Furthermore, such division also reduces noisecaused by reading the dust.

FIG. 5 is a plan, rear view of the platen. With reference to the figure,platen 205 has one end with a mark 205A having a single color and ageometry having in the main scanning direction a length varyingdepending on the position in the sub scanning direction. In thisdescription, mark 205A is a black triangle. Furthermore, mark 205A hasone side arranged parallel to one side of platen 205.

Reader 213 or a sensor provided separate from reader 213 and fixed tomain body 103 can be used to detect the length of mark 205A in the mainscanning direction to detect the position of platen 205 relative toreader 213.

FIG. 6 shows a location on platen 205 read by reader 213. Reader 213 hasline sensors 213R, 213G and 213B having filters of red (R), green (G)and blue (B), respectively, arranged in a direction in which an originalis transported D1, red first, followed by green and then blue.

Line sensors 213R, 213G and 213B receive light transmitted throughplaten 205 at regions 205R, 205G and 205B, respectively. Regions 205R,205G and 205B are arranged to be spaced by three lines. The originalinitially moves fast region 205R, then region 205G and finally region205B. As such, light reflected by the original at a location isinitially received by the red light receiving line sensor 213R, then thegreen light receiving line sensor 213G, and finally the blue lightreceiving line sensor 213B. Line sensors 213R, 213G, 213B spaced bythree lines thus will not simultaneously receive light reflected by theoriginal at a single location.

If platen 205 has adhering thereto dust 300 having a maximal length ofat most four lines, then dust 300 will not exist at two or more ofregions 205R, 205G, 205B concurrently as platen 205 moves oscillatingparallel to the sub scanning direction. FIG. 6 shows a case where dust300 exists at region 205G. In this case, light reflected by dust 300 isreceived only by line sensor 213G and not received by line sensor 213Ror 213B.

Furthermore, as platen 205 oscillates, dust 300 will exists at differentregions. More specifically, when platen 205 moves in direction D1, dust300 initially exists at region 205R, then region 205G and finally region205B. In contrast, when platen 205 moves in a direction oppositedirection D1, dust 300 exists initially at region 205B, then region205G, and finally region 205R.

As such, light reflected by dust 300 is received in such an order thatwhen platen 205 moves in direction D1 the light is received initially byline sensor 213R, then line sensor 213G and finally line sensor 213B andwhen platen 205 moves opposite to direction D1 the light is receivedinitially by line sensor 213B, then line sensor 213G, and finally linesensor 213R.

FIG. 7 is a block diagram showing a configuration of the image processorof the image reading apparatus in the present embodiment. With referenceto the figure, image processor 215 receives R, G and B signals fromreader 213. Image processor 215 includes an analog/digital (A/D)converter 251 receiving an analog signal from reader 213 to convert theanalog signal to a digital signal, a shading corrector 253 correctinguneven illumination provided by the source of light 206 or the like, aninterline corrector 255 synchronizing the R, G and B signals to be asingle line of an original, a color aberration corrector 257 correctingdistortion in the main scanning direction introduced by lens 211, anoise detection processor 259 detecting noise from each of the R, G andB signals, a noise corrector 260 effecting a process to correct a noisepixel, a controller 263 generally controlling image processor 215, and aprinter interface 261 used to output an image to image forming apparatus20. Controller 263 has a position detector 265 connected thereto todetect the position of platen 205. Position detector 265 detects alength of mark 205A of platen 205 in the main scanning direction.

Interline corrector 255 delays the R and G signals by eight and fourlines, respectively, to synchronize the R, G and B signals to be asingle line of the original, since as has been described previously,line sensors 213R, 213G, 213B are spaced in the sub scanning directionby a distance corresponding to three lines.

Noise detection processor 259 receives the R, G and B signals from coloraberration corrector 257 and from controller 263 the position of platen205 and a direction in which platen 205 moves. Noise detection processor259 detects a noise pixel for each of the R, G and B signals receivedfrom color aberration corrector 257, and outputs to noise corrector 260and controller 263 logical signals of “1” and “0” indicating a noisepixel and a pixel other than a noise pixel, respectively. The detailwill be described later.

Noise corrector 260 receives the R, G and B signals from coloraberration corrector 257 and from noise detection processor 259 receivesfor each of the R, G and B signals logical signal of “1” and “0”indicating a noise pixel and a pixel other than a noise pixel,respectively.

Noise corrector 260 determines from logical signals corresponding to theR, G and B signals, respectively, a color of a pixel determined as anoise pixel. More specifically, noise corrector 260 determines a colorof a noise pixel successive in the sub scanning direction. Furthermore,if noise pixels are not successive in the sub scanning direction then acolor of a pixel existing between two noise pixels is determined, and ifthe pixels are identically located in the main scanning direction andvary in color in the sub scanning direction in the following order:

(1) CBMRY or YRMBC

(2) CBKRY or YRKBC

(3) RYGCB or BCGYR

(4) RYWCB or BCWYR

then the pixels are all determined as noise pixel, wherein R, G, B, C,M, Y, K, and W represent red, green, blue, cyan, magenta, yellow, black,and white, respectively. It should be noted, however, that herein anorder in which a color varies is only indicated, and two or more pixelsof the same color may be successively provided. For example, it may beCCBBMMRRYY.

Thus if dust has a size read by a plurality of line sensorsconcurrently, herein a size corresponding to four or more lines, noisecaused by reading the dust can be detected.

Furthermore, noise corrector 260 operates for each of the R, G and Bsignals in response to a logical signal corresponding thereto to replacea value of a pixel determined as a noise pixel with that of aneighboring, non-noise pixel. This can simply be done by replacing thevalue of the pixel determined as the noise pixel with an average,maximum or minimum value of a plurality of neighboring non-noise pixels.Noise corrector 260 outputs to the printer interface the R, G and Bsignals with any noise pixels replaced with a neighboring pixel(s).

Controller 263 receives the position of platen 205 from positiondetector 265 and from noise detection processor 259 logical signals of“1” and “0” indicating a noise pixel and a pixel other than noise pixel,respectively. Controller 263 determines from these signals the dust'slocation on platen 205. More specifically, it determines the position ofplaten 205 in the sub scanning direction from the position of platen 205and a logical signal's line number, and the position of platen 205 inthe main scanning direction from a location of a noise pixel of thelogical signal.

The noise detection process will more specifically be describedhereinafter. As has been described with reference to FIG. 6, linesensors 213R, 213G and 213B will read different locations on an originalat the same timing. Interline corrector 255 synchronizes the R, G and Bsignals' lines to obtain R, G and B signals having read a singlelocation on the original.

As such, if platen 205 has dust adhering thereon, R, G and B signalshaving read a single location on an original have one of them affected.

FIGS. 8A and 8B represent an example of RGB signal output from thereader. FIG. 8A shows an example of reading a white area of an originalwith black dust adhering on the platen's region 205R corresponding toline sensor 213R. Line sensor 213R reads a portion of the original withthe black dust on region 205R. Subsequently, the portion of the originalmoves to regions 205G, 205B corresponding to line sensors 213G, 213B,when the dust does not exist on regions 205G, 205B, since the originaland platen 205 moves at different rates. As such line sensors 213G, 213Bwill read the original's white area. Consequently, only an R signaloutput from line sensor 213R is reduced in lightness and line sensors213G, 213B output G and B signals high in lightness. Note that herein,“lightness” indicates a value output from the three line sensors 213R,213G, 213B corresponding to a reflection of light.

The FIG. 8A RGB signals' combination is seldom output when an originalis read without dust adhering thereto. A combination closest thereto isa case where an area of cyan, a color complementary to red, is read.FIG. 8B represents RGB signal output from reader 213 when an original'scyan area is read. The R signal significantly drops in lightness, andthe G and B signals also drops in lightness.

The FIG. 8A RGB signal and the FIG. 8B RGB signal are significantlydifferent in whether the B and G signals are affected. By detecting thisdifference, black dust can be detected as noise without detecting a cyanline erroneously as noise. As such, the threshold value for detectingvariation in lightness can simply be provided by the smallest one of thefollowing values.

(1) Detecting Dust of Achromatic Color High in Lightness

To prevent a cyan line from being detected erroneously as noise, thedifference between a maximum value in lightness (255) and one of thevalues in lightness output from the line sensors other than line sensor213R, i.e., line sensors 213G and 213B, reading a color complementary tored, or cyan, can be set as Ref1(G), Ref1(B). To prevent a magenta linefrom being detected erroneously as noise, the difference between themaximum value in lightness (255) and one of the values in lightnessoutput from the line sensors other than line sensor 213G, i.e., linesensors 213R and 213B, reading a color complementary to green, ormagenta, can be set as Ref1(R), Ref1(B). To prevent a yellow line frombeing detected erroneously as noise, the difference between the maximumvalue in lightness (255) and one of the values in lightness output fromthe line sensors other than line sensor 213B, i.e., line sensors 213Rand 213G, reading a color complementary to blue, or yellow, can be setas Ref1(R), Ref1(G).

(2) Detecting Dust of Achromatic Color Low in Lightness

To prevent a red line from being detected erroneously as noise, thedifference between a minimum value in lightness (0) and one of thevalues in lightness output from the line sensors other than line sensor213R, i.e., line sensors 213G and 213B, reading red color, can be set asRef1(G), Ref1(B). To prevent a green line from being detectederroneously as noise, the difference between the minimum value inlightness (0) and one of the values in lightness output from the linesensors other than line sensor 213G, i.e., line sensors 213R and 213B,reading green color, can be set as Ref1(R), Ref1(B). To prevent a blueline from being detected erroneously as noise, the difference betweenthe minimum value in lightness (0) and one of the values in lightnessoutput from the line sensors other than line sensor 213B, i.e., linesensors 213R and 213G, reading blue color, can be set as Ref1(R),Ref1(G). Thus more than one threshold value Ref1(R), Ref1(G), Ref1(B)are obtained, and a minimum value thereof can simply be used.

While herein black dust is detected as noise, dust of achromatic colorother than black can also be detected, since any achromatic dust affectsall of R, G and B signals.

Furthermore, while herein a white original is read by way of example, anoriginal of any color other than white may be read.

FIG. 9 is a block diagram showing a configuration of the noise detectionprocessor of the image reading apparatus in the present embodiment. Withreference to the figure, noise detection processor 259 includeslightness difference detectors 301R, 301G, 301B extracting from R, G andB signals, respectively, a region having a predetermined feature,detection result extension processors 303R, 303G, 303B extending theregion extracted by lightness difference detectors 301R, 301G, 301B to avicinity thereof, NOR devices 305R, 305G, 305B, AND devices 307R, 307G,307B, and detected-area extension processors 309R, 309G, 309B.

R, G, B signals are input to noise detection processor 259, one line ata time, sequentially. Note that the R, G and B signals may be inputcollectively by a plurality of lines or an entire image.

Lightness difference detector 301R receives the R signal and thresholdvalue Ref1(R) and extracts from the R signal a region having thepredetermined feature. This region is a region having a limitedvariation in lightness and a difference in lightness of at leastthreshold Ref1(R) from a region surrounding it. Such region is onlyrequired to have a size of at least one pixel. In this description apixel included in a region having the predetermined feature will bereferred to as a feature pixel.

The region having the predetermined feature may be extracted byemploying an edge extraction filter. More than one edge extractionfilter are prepared for sizes of edge regions, respectively, and a valueobtained as a result of filtering is compared with threshold valueRef1(R). A pixel satisfying a condition with threshold value Ref1(R) isdetermined as a center pixel of an edge region and from an edgeextraction filter satisfying that condition the edge region's size isobtained.

FIGS. 10A-10F represent the edge extraction filter by way of example.FIG. 10A represents an edge extraction filter used to detect an edgeregion of a size of one pixel when an R signal is input, one line at atime. FIG. 10B represents an edge extraction filter used to detect anedge region of a size of one pixel when an R signal is input in aplurality of lines correctively.

FIG. 10C represents an edge extraction filter used to detect an edgeregion of a size of three pixels when an R signal is input, one line ata time. FIG. 10D represents an edge extraction filter used to detect anedge region of a size of three pixels when an R signal is input in aplurality of lines correctively.

FIG. 10E represents an edge extraction filter used to detect an edgeregion of a size of five pixels when an R signal is input, one line at atime. FIG. 10D represents an edge extraction filter used to detect anedge region of a size of five pixels when an R signal is input in aplurality of lines correctively.

These edge extraction filters are established under the followingconditions:

(1) An edge region high in lightness is extracted when an average inlightness of pixels A and B minus that in lightness of pixel C equals atleast threshold value Ref1(R):(Average of Pixels A and B)−(Average of Pixel C)>Ref1(R).

In that case, the center pixel is one of pixels A, B and C that is thehighest in lightness.

(2) An edge region low in lightness is extracted when an average inlightness of pixel C minus that in lightness of pixels A and B equals atleast threshold value Ref1(R):(Average of Pixel C)−(Average of Pixels A and B)>Ref1(R).

In that case, the center pixel is one of pixels A, B and C that is thelowest in lightness.

G and B signals can also be handled with an edge extraction filtersimilar to that used for the R signal.

With reference again to FIG. 9, the feature pixel extracted by lightnessdifference detector 301R is represented by a logical signal of “1” and apixel other than the feature pixel is represented by a logical signal of“0” and thus output to AND device 307R and detection result extensionprocessor 303R.

Detection result extension processor 303R sets a pixel neighboring thefeature pixel extracted by lightness difference detector 301R as afeature pixel to extend a region having the predetermined feature. Inother words, a pixel that exists in a vicinity of a pixel of “1” invalue as represented by a logical signal received from lightnessdifference detector 301R and has a value of “0” is changed to “1”. Noisecan be detected with higher precision. A logical signal havingcontributed to extended region is output to NOR devices 305G, 305B.

Lightness difference detector 301G receives the G signal and thresholdvalue Ref1(G) and extracts from the G signal a region having thepredetermined feature. This region is a region having a limitedvariation in lightness and a difference in lightness of at leastthreshold Ref1(G) from a region surrounding it.

The feature pixel extracted by lightness difference detector 301G isrepresented by a logical signal of “1” and a pixel other than thefeature pixel is represented by a logical signal of “0” and thus outputto AND device 307G and detection result extension processor 303G.

Detection result extension processor 303G sets a pixel neighboring thefeature pixel extracted by lightness difference detector 301G as afeature pixel to extend a region having the predetermined feature. Alogical signal having contributed to an extended region is output to NORdevices 305R, 305B.

Lightness difference detector 301B receives the B signal and thresholdvalue Ref1(B) and extracts from the B signal a region having thepredetermined feature. This region is a region having a limitedvariation in lightness and a difference in lightness of at leastthreshold Ref1(B) from a region surrounding it.

The region having the predetermined feature may be extracted byemploying an edge extraction filter. More than one edge extractionfilter are prepared for sizes of edge regions, respectively, and a valueobtained as a result of filtering is compared with threshold valueRef1(B). A pixel satisfying a condition with threshold value Ref1(B) isdetermined as a center pixel of an edge region and from an edgeextraction filter satisfying that condition the edge region's size isobtained.

The feature pixel extracted by lightness difference detector 301B isrepresented by a logical signal of “1” and a pixel other than thefeature pixel is represented by a logical signal of “0” and thus outputto AND device 307B and detection result extension processor 303B.

Detection result extension processor 303B sets a pixel neighboring thefeature pixel extracted by lightness difference detector 301B as afeature pixel to extend a region having the predetermined feature. Alogical signal having contributed to an extended region is output to NORdevices 305R, 305G.

NOR device 305R receives from each of detection result extensionprocessor 303G, 303B a logical signal having contributed to an extendedregion. NOR device 305R outputs to AND device 307R a logical signalcorresponding to an inversion of an OR of two received logical signals.More specifically, a pixel which is not a feature pixel for either a Gor B signal is represented by a logical signal of “1” for output and apixel which is a feature pixel for at least one of the signals isrepresented by a logical signal of “0” for output.

AND device 307R outputs to detected-area extension processor 309R an ANDof a logical signal received from lightness difference detector 301R andthat received from NOR device 305R. More specifically, a pixel which isa feature pixel for an R signal and not an extended feature pixel foreither a B or G signal is represented by a logical signal of “1” and apixel different therefrom is represented by a logical signal of “0” foroutput. A pixel of “1” in value as represented by this logical signalindicates a noise pixel. Thus by NOR device 305R and AND device 307R afeature pixel extracted from an R signal that has not been extracted asa feature pixel for either a G or B signal is determined as a noisepixel.

If detected-area extension processor 309R receives a logical signal of“1” from AND device 307R for a pixel, detected-area extension processor309R sets a pixel that exists in a vicinity of the pixel correspondingto the “1” to a “1” to extend a noise pixel's range. This is done toprovide improved precision with which a noise pixel is corrected. Thenoise pixel extended in range is represented by a logical signal of “1”which is in turn output to noise corrector 260.

NOR device 305G receives from each of detection result extensionprocessors 303R, 303B a logical signal having contributed to an extendedregion. NOR device 305G outputs to AND device 307G a logical signalcorresponding to an inversion of an OR of two received logical signals.More specifically, a pixel which is not a feature pixel for either an Ror B signal is represented by a logical signal of “1” for output and apixel which is a feature pixel for at least one of the signals isrepresented by a logical signal of “0” for output.

AND device 307G outputs to detected-area extension processor 309R an ANDof a logical signal received from lightness difference detector 301G andthat received from NOR device 305G. More specifically, a pixel which isa feature pixel for a G signal and not an extended feature pixel foreither a R or B signal is represented by a logical signal of “1” and apixel different therefrom is represented by a logical signal of “0” foroutput. A pixel of “1” in value as represented by this logical signalindicates a noise pixel. Thus by NOR device 305G and AND device 307G afeature pixel extracted from a G signal that has not been extracted as afeature pixel for either an R or B signal is determined as a noisepixel.

If detected-area extension processor 309G receives a logical signal of“1” from AND device 307G for a pixel, detected-area extension processor309G sets a pixel that exists in a vicinity of the pixel correspondingto the “1” to a “1” to extend a noise pixel's range. This is done toprovide improved precision with which a noise pixel is corrected. Thenoise pixel extended in range is represented by a logical signal of “1”which is in turn output to noise corrector 260.

NOR device 305B receives from each of detection result extensionprocessors 303R, 303G a logical signal having contributed to an extendedregion. NOR device 305B outputs to AND device 307B a logical signalcorresponding to an inversion of an OR of two received logical signals.More specifically, a pixel which is not a feature pixel for either an Ror G signal is represented by a logical signal of “1” for output and apixel which is a feature pixel for at least one of the signals isrepresented by a logical signal of “0” for output.

AND device 307B outputs to detected-area extension processor 309B an ANDof a logical signal received from lightness difference detector 301B andthat received from NOR device 305B. More specifically, a pixel which isa feature pixel for a B signal and not an extended feature pixel foreither an R or G signal is represented by a logical signal of “1” and apixel different therefrom is represented by a logical signal of “0” foroutput. A pixel of “1” in value as represented by this logical signalindicates a noise pixel. Thus by NOR device 305B and AND device 307B afeature pixel extracted from a B signal that has not been extracted as afeature pixel for either an R or G signal is determined as a noisepixel.

If detected-area extension processor 309B receives a logical signal of“1” from AND device 307B for a pixel, detected-area extension processor309B sets a pixel that exists in a vicinity of the pixel correspondingto the “1” to a “1” to extend a noise pixel's range. This is done toprovide improved precision with which a noise pixel is corrected. Thenoise pixel extended in range is represented by a logical signal of “1”which is in turn output to noise corrector 260.

Thus the image reading apparatus 10 noise detection processor 259extracts a feature pixel from each of R, G and B signals output from thethree line sensors 213R, 213G, 213B, and sets as a noise pixel a featurepixel extracted from one of R, G and B signals. Thus noise caused bydust existing on a platen can be detected from an image obtained byreading an original.

Noise Detection Processor in Exemplary Variation

When dust of achromatic color reflects light, the light is received byline sensors 213R, 213G, 213B at different times. The line sensors,however, should output lightness close in value.

In this exemplary variation if pixels detected from R, G and B signalsas noise pixels, respectively, have a large difference in lightness thedetection is cancelled.

FIG. 11 is a block diagram showing a configuration of the noisedetection processor in the exemplary variation. It is different from theFIG. 9 noise detection processor 259 in that first delay circuits 311R,311G, 311B, a lightness comparison processor 313, second delay circuits315R, 315G, 315B, and detection result canceller 317 are additionallyintroduced. The remainder has the same configuration as the FIG. 9 noisedetection processor.

The first delay circuits 311R, 311G, 311B receive from controller 263 adirection in which platen 205 moves, and delays R, G and B signals sothat a corresponding noise pixel to have an identical position. Thenumber of lines to be delayed depends on in which direction the platenmoves.

In Case with Platen Moving in Direction Opposite to That of Original

As has been described previously, a noise pixel is detected first for aB signal, then a G signal and finally for an R signal. Accordingly, a Bsignal of a line including a detected noise pixel is delayed until aline including a noise pixel of an R signal that corresponds to thenoise pixel of the B signal is output. Furthermore, a G signal of a lineincluding a detected noise pixel is delayed until a line including anoise pixel of an R signal that corresponds to the noise pixel of the Gsignal is output.

The first delay circuit 311B delays a B signal by a number of lines tobe delayed as determined by the following equation (1):B signal's number of lines to be delayed=(line interval×(systemrate/platen movement rate)+line interval)×2  (1),wherein the system rate indicates a rate at which an original istransported, and the line interval is an interval of line sensors 213R,213G, 213B (unit: line).

The first delay circuit 311G delays a G signal by a number of lines tobe delayed as determined by the following equation (2):G signal's number of lines to be delayed=(line interval×(systemrate/platen movement rate)+line interval)  (2).

Note that noise's length (unit: line) is represented by the followingequation (3):Noise's length=dust's size×(system rate/platen movement rate)  (3).

In Case with Platen Moving in the Same Direction as Original

As has been described previously, a noise pixel is detected first for aB signal, then a G signal and finally for an R signal. Accordingly, an Rsignal of a line including a detected noise pixel is delayed until aline including a noise pixel of a B signal that corresponds to the noisepixel of the R signal is output. Furthermore, a G signal of a lineincluding a detected noise pixel is delayed until a line including anoise pixel of a B signal that corresponds to the noise pixel of the Gsignal is output.

The first delay circuit 311R delays an R signal by a number of lines tobe delayed as determined by the following equation (4):R signal's number of lines to be delayed=(line interval×(systemrate/platen movement rate)−line interval)×2  (4).

The first delay circuit 311G delays a G signal by a number of lines tobe delayed as determined by the following equation (5):G signal's number of lines to be delayed=(line interval×(systemrate/platen movement rate)−line interval)  (5).

The second delay circuits 315R, 315G, 315B delay logical signalsreceived from AND circuits 307R, 307G, 307B, respectively. The seconddelay circuits 315R, 315G, 315B delay the same number of lines as thefirst delay circuits 311R, 311G, 311B.

Lightness comparison processor 313 receives delayed R, G and B signalsfrom the first delay circuits 311R, 311G, 311B, and threshold valueRef2. Lightness comparison processor 313 outputs to detection resultcanceller 317 a logical signal of “1” if a corresponding pixel's maximaland minimal values in lightness has a difference of at least thresholdvalue Ref2, otherwise lightness comparison processor 313 outputs alogical signal of “0” to detection result canceller 317. A logicalsignal with this value being “1” is referred to as a cancel signal.

Detection result canceller 317 receives from the second delay circuits315R, 315G, 315B a logical signal of “1” representing a noise pixel, andreceives from lightness comparison processor 313 a logical signalincluding the cancel signal. Detection result canceller 317 sets as “0”a logical signal of “1” representing a noise pixel received from each ofthe second delay circuits 315R, 315G, 315B when a corresponding logicalsignal received from lightness comparison processor 313 is a cancelsignal of “1”. More specifically, pixels determined as noise pixel foreach of R, G and B signals. If the pixels corresponding to each otherhave difference in lightness exceeding a predetermined value, the pixelsare not determined as a noise pixel. A noise pixel can thus be detectedwith higher precision.

Note that while the present embodiment has been described with reader213 fixed to main body 103 by way of example, alternatively, the presentinvention is also applicable to moving reader 213 for scanning. Forexample, the upper restraint plate is of monochromatic color of white orblack, and reader 213 or the source of light 206, reflector mirror 209and reflector member 208 are moved in the sub scanning direction forscanning. During the scan, platen 205 can be oscillated in the subscanning direction to detect dust adhering on platen 205.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. An image reading apparatus comprising: at least three line sensorsmutually spaced in a sub scanning direction to scan an original in thesub scanning direction, wherein outputs of the at least three linesensors are composited together to form an image of the original; aplaten arranged between the original and said at least three linesensors; a mover moving said platen at a rate relative to said at leastthree line sensors, said rate being different from a rate of movement ofthe original relative to said at least three line sensors; an extractorconfigured to extract a feature pixel having a predetermined featurefrom any of at least three data output corresponding to said at leastthree line sensors; and a detector configured to identify a noise pixelby comparing said at least three data corresponding to a single locationon the original and identifying the feature pixel as noise pixel if saidfeature pixel is detected for one of said at least three data and is notdetected as a feature pixel for each of the other at least three data,wherein said extractor includes a region extractor extracting a regionhaving a limited variation in lightness and a difference from aneighboring region in lightness of at least a predetermined value, andextracts as said feature pixel a pixel included in a region extracted.2. The image reading apparatus of claim 1, further comprising aninterline corrector synchronizing said at least three data output bysaid at least three line sensors to be values of pixels reading a singlelocation on the original, wherein said at least three data synchronizedby said interline corrector are inputted, one line at a time,sequentially.
 3. The image reading apparatus of claim 1, wherein saiddetector further includes an extender setting as a feature pixel a pixelneighboring said feature pixel, and detects said feature pixel extractedfrom one of said at least three data, as a noise pixel if said featurepixel is not a feature pixel extended by said extender for said at leastthree data other than said one of said at least three data.
 4. The imagereading apparatus of claim 1, wherein said at least three sensors eachinclude a filter different in spectral sensitivity to receive lightreflected from the original through said filter.
 5. The image readingapparatus of claim 1, further comprising an original transportertransporting the original while said at least three line sensors scanthe original.
 6. The image reading apparatus of claim 1, wherein saidmover oscillates said platen in the sub scanning direction.
 7. The imagereading apparatus of claim 1, further comprising a determinerdetermining a pixel to be corrected, said pixel to be corrected being anoise pixel detected by said detector and a pixel neighboring said noisepixel detected.
 8. The image reading apparatus of claim 1, furthercomprising: a position detector detecting said platen's position; and alocation determiner determining a location of a source of noise on saidplaten from the position detected by said position detector and alocation of said noise pixel detected by said detector.